The present invention relates to a method for the production of foamed polystyrene films as used, for instance, for the manufacture of shaped bodies, i.e. cups, food containers or the like, but also for the production of labels. In particular, the invention relates to a method of producing a multi-layer preferably two-layer laminate of a foamed polystyrene film with a co-extruded, preferably unfoamed film, especially for further working into labels or shaped bodies, utilizing the shrinkability capacity of the films. In this connection, a polystyrene resin composition is extruded together with nitrogen or carbon dioxide as expansion agents and a pore-former from the nozzle of an extruder and the extruded mass is inflated before cooling to below the softening point.
One method for the manufacture of the polystyrene films indicated above is known from European Patent 55437. In it, carbon dioxide and nitrogen are mentioned in each case as possible expansion agents. The films produced are, however, unsatisfactory, particularly for the manufacture of labels. A relatively rough surface and thus poor printability result. The further working into containers can also only be poorly effected with such a film. A certain stiffness of the film and a thickness of film which can be set within the narrowest possible limits is necessary.
Starting from the above prior art, the object of the present invention is so to development the method for the production of foamed polystyrene films as to result in a production of foamed polystyrene films which is economical as to material and as unsusceptible as possible to disturbances and which also is as insensitive as possible to aging and can be readily printed.